1. Field of the Invention
The present invention relates to optical detection means, such as optical scanner or pick-up systems in CD (compact disc) and DVD (digital versatile disc) equipment or optical receivers in optical transmission lines.
2. Description of Prior Art
Driven by the increasing multi-media demands with regard to photo, audio and video, the memory requirement is currently growing at an exponential rate. Optical memory systems here constitute a particularly attractive solution between magnetic tape storage media and hard discs. With access times in the range of milliseconds, which are only slightly higher than that of hard discs, optical memory systems offer the possibility of day-to-day use on the part of the end user. With tape drives, they share the property of archivability, i.e. the property that the storage medium may be replaced in the drive.
Current memory capacities of about 5 gigabytes and short-term prognoses of up to 50 gigabytes render optical storage media competitive in the area of video-recorders as well, so that it seems certain that this sector will see strong growth. All these optical memories have in common that both the writing operation and the readout of the data stored on the storage medium is performed by means of a light beam, and that in the readout a light beam modified by the storage medium in accordance with the data stored thereon, e.g. a light beam which is reflected, transmitted, or polarized, is converted to an electrical signal by an opto-electronic detector.
Further fields of application of such opto-electronic detectors are, for example, the area of optical transmission, such as in the form of a detector array with parallel transmission lines.
With all these applications for optical detectors there is a problem of fixedly mounting the optical detector in relation to the light beam to be detected so that the light beam is in optimum alignment with the optical detector. This requires, e.g., time-consuming and expensive, since precise, adjustment in the assembly of the optical device wherein the optical detector is used, or requires time-consuming and costly precautions to be taken against thermal drifts or other application-specific operational incidents, such as precautions against misalignment of the light beam relative to the optical detector, or other factors negatively influencing the readout.
An example of such optical devices are CD and DVD devices. They include a reading head, the task of which is to scan and retrieve the data from the CD or DVD storage medium. In addition to a laser diode, optics of optical devices, focus control and tracking control actuators, the reading head also includes a photodiode matrix which provides, in addition to data readout, signals for focus and tracking control. A photodiode matrix of a conventional structure is shown, e.g., in FIG. 12 and has been described, for example, in the article by M. O. Freeman et al. entitled “Robust Focus and Tracking Detection for Holographic Digital Versatile Disc Optical Pickup-Head Modules” in Jpn. Journal of Applied Physics, 1999, pp. 1755–1760. The diode matrix shown in FIG. 12 consists of four reception diodes 900, 902, 904 and 906, designated by A, B, C and D and arranged in a planar 2×2 matrix. The diode array of FIG. 12 is arranged in the reading head of the CD or DVD device so as to receive a light beam reflected from a track on the CD or DVD. In the case of a CD, the data is stored in the track of the optical disc, e.g., by a predetermined sequence of planar locations and elevations (or depressions, depending on the point of view) (so-called pits). The light beam incident on and reflected from the plate is wider than the track or the pits, so that in the reflection of the light beam focused thereon, the elevations lead to a destructive interference of the reflected light beam, so that the elevations will be recognized by means of reduced light intensity of the reflected beam, and planar locations may be recognized by an increased, or maximum, light intensity. The optics map the reflected light beam onto the diode array. From the output signals of reception diodes 900–906, the data signal from which the data stored in the track is obtained is obtained by summing up all output signals of these diodes, i.e. by offsetting A+B+C+D. As a result of the constantly increasing speed of the readout and the increasing data density, the reception diodes 900–906 must be diodes suitable for rapid readout. Such diodes will be referred to as HF diodes. The reason why more than one, i.e. four HF diodes 900–906 are provided is that the intensity distribution information obtainable from these HF diodes is used for focus and tracking control. For focus control, for example, the light beam focused onto the optical plate and reflected back therefrom is mapped via an astigmatic, e.g. cylindrical, lens onto the diode array of FIG. 12. The target distance between reading head and optical plate is specified such that a circular mapping of the light beam focused onto the optical plate results on the diode array of FIG. 12. In the event of defocusing, an elliptical deformation of the light spot mapped onto the diode array of FIG. 12 results in one of the main axes of the astigmatic lens. For distance and/or focus control, the output signals of diodes 902 and 904 and of diodes 900 and 904 are therefore added together, and the sums are subtracted from each other, i.e. (A+C)−(B+D), to detect the elliptical deformation in the event of defocusing, it being assumed that the main axes of the astigmatic lens extend along the axes x and y indicated in FIG. 12. When the astigmatic lens is aligned with the strongly refractive axis along axis x, an elliptical extension along the main axis y indicates, e.g., a distance larger than the target distance, whereas an elliptical extension along axis x indicates, e.g., too small a distance. In accordance with the focus control signal, the distance of the objective lens from the optical plate may then be adjusted by an actuator. Similarly, the signal (A+D)−(C+B) may be used for controlling tracking of the laser on the data carrier, i.e. for radially moving the reading head across the revolving optical plate. This linkage of the diodes is suitable to detect the so-called push-pull pattern caused by interference of orders of refraction generated by the elevations acting like a phase grating.
Another possibility of providing a signal for tracking control, i.e. for setting the radial position of the reading head, or the position of the reading head in the direction transverse to the track, is to arrange two additional reception diodes outside the actual HF diode array. Such a structure may be seen in FIG. 13. As can be seen, two reception diodes 908 and 910 are provided in addition to the four HF diodes 900–906. Since the tracking control requires a sampling and/or readout rate which is not too high, these reception diodes are read out less rapidly, detection diodes for slower readout being referred to as LF diodes below. These LF diodes 908 and 910 are provided to receive two light beams which are focused onto the optical plate and are reflected, in addition to the actual data readout light beam. These additional light beams are obtained, for example, from the actual laser diode being used for data readout by detraction as the first-order detraction and are arranged, in relation to the main beam, such that they are focused onto the optical plate slightly ahead of and slightly behind the main light beam in relation to the direction of the track and in a manner which is somewhat transverse to the direction of the track. In the event that the reading head is optimally aligned with the track, both additional beams of light are modulated by the pits of the track due to the finite extension of the focused additional light beams. In the event of a lateral track misalignment of the reading head, only the finite extension of one of the additional light beams is in the track, whereas the other additional light beam is not modulated. From this information, tracking control signals for radially controlling the reading head may be derived.
Both diode arrays of FIGS. 12 and 13 have in common that they are sensitive towards non-ideal positioning of the light beam due to misadjustments or thermal drifts. In addition, optical offsets due to optical variations in the substrate thickness on the storage medium and tilts of the optical plate from a target position have a negative impact on the error rate of the data retrieved, since these events cannot be detected. The result is a time-consuming and costly manufacturing process eliminating these sources of error by means of precise adjustment, expensive components and more costly tracking and focus control.
Additional problems result from the constantly increasing requirements placed upon the storage density and compactness of the optical storage plate. The finite extension of the laser spot causes, along with increasing storage density, crosstalk phenomena due to the simultaneous readout of two adjacent tracks on the data carrier. Previous detectors have offered no possibility of detecting this. Again, this results in increased error rates, since, for example, a pit which may be present on the adjacent track may lead to a false readout result if no pit is present on the track to be read out, and at the same time this restricts the track pitch on the data carrier, which, in turn, plays a decisive role in determining the storage capacity of the medium.
In addition, a disadvantage of the previous diode structures is the high expenditure required to obtain information from the light beam received about a misalignment, and/or correction signals for the latter. Subdividing the reception window into four parts, i.e. into the HF diodes 900–906, quadruples, e.g., the readout expenditure of these diodes, which for each of these diodes must evidently be designed for readout speeds suitable for reading out data. In the case of FIG. 13, the LF diodes 908 and 910 used for tracking control are based on the prerequisite of the additional optical expenditure of two beams of first-order defraction.